Although commercial application of ultrasonic holography as been accurately pursued by many persons in the scientific and industrial communities for many years, only limited results have been obtained even though it was once thought that ultrasonic holography held great promise. It was felt that the application of ultrasonic holography was particularly applicable to the fields of nondestructive testing of materials and medical diagnostics of soft tissues that are relatively transparent to ultrasonic radiation. One of the principal problems that has been encountered and not effectively resolved is the difficulty of obtaining quality and consistent images. A further problem has been the inability to vary the magnification of the image while maintaining a consistent high quality image.
As background, FIG. 1 shows a typical "real time" prior art ultrasonic holographic imaging system generally designated with the numeral 10. The system 10 is intended to ultrasonically inspect the interior of an object 12 such as the soft tissue of a human limb. The ultrasonic holographic imaging system 10 generally has a hologram generating subsystem 14 for generating an ultrasonic hologram. The system 10 also includes a hologram viewing subsystem (optical-subsystem) 16 for optically viewing the interior of the object 12 from a first order diffraction from the formed ultrasonic hologram.
The subsystem 14 includes an object ultrasonic transducer 18 for generating plane waves through a liquid or gel coupling medium 20 contained in a deformable membrane 22. The deformable membrane 22 intimately contacts the object 12 on one side and a deformable membrane 24 contacts the object on the other side to provide ultrasonic coupling with minimum energy loss or wave distortion. The deformable membrane 24 forms part of the side wall of a container 28 that contains a liquid coupling medium 30.
One of the principal components and the main concern of this invention is the provision of an ultrasonic imaging lens system 32 for viewing a large field and focusing at a desired object focal plane 34. The prior art ultrasonic imaging lens system 32 focuses the ultrasonic energy onto a hologram detector surface 36. The ultrasonic imaging lens system 32 includes a large diameter object lens 38 that is moveable with respect to a large diameter lens 40 for moving the lens to different desired focal planes 34 in the object 12. The lens 40 is stationary and is positioned at a fixed focal length from the detector surface 36. The ultrasonic imaging lens system 32 includes a mirror 41 for reflecting the ultrasonic energy approximately 90.degree. and onto the hologram detection surface 36 to form the hologram.
A ultrasonic reference transducer 42 directs coherent ultrasonic plane waves through the liquid medium 30 at an off-axis angle to the hologram detector surface 36 to form the hologram. Preferably, the hologram detection surface 36 is the liquid/gas interface surface that is supported in an isolated dish or mini-tank 44.
The hologram viewing subsystem 16 includes an optical lens 45 to achieve an effective point source of a coherent light beam from a laser (not shown). The focused coherent light is reflect from a mirror 46 through a collimating optical lens 47 and then onto the hologram detector surface 36 to illuminate the hologram and generate diffracted optical images. The reflected coherent light radiation containing holographic information is directed back through the collimating lens 47 and separated into precisely defined diffracted orders in the focal plane of the collimating lens 47. A filter 48 is used to block all but a first diffracted order from a viewing lens 49 to enable a human eye, a photographic film or a video camera to record in "real time" the object at the object focal plane. As previously mentioned, although such a system is operable, it has been difficult to obtain quality and consistent images.
A prior art ultrasonic lens system, similar to that described, is presented in U.S. Pat. No. 3,802,533 entitled "Improvements In and Relating To Ultrasonic Lenses" granted to Byron B. Brenden. Such patent is principally directed to the specific structure of the ultrasonic lenses.
One of the principal objects and advantages of this invention is to provide an improved ultrasonic holographic imaging apparatus that has the ability to change the magnification of the image relative to the object as well as change the focus to different planes in the object while maintaining consistent high quality images.
These and other objects and advantages of this invention will become apparent upon reading the following detailed description of a preferred embodiment.